Typically, Separation Agreements do not expire. However, there may be certain provisions contained in a separation agreement which do expire. For instance, a separation agreement may have a non-compete or non-solicitation provision which expire after a period of time. Confidentiality provisions typically do not expire, but it really depends on what you specific contract says. Speaking to the plaintiff's attorney may not be a violation of your agreement, unless he/she is asking you to divulge confidential or proprietary information held by your former employer.
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You should speak with a local employment attorney about the lawsuit (whatever information you have about it) and also have the attorney review your separation agreement.
You can search for an attorney on Avvo or contact the National Employment Lawyers Association for a referral.
This answer is provided for guidance only. DO NOT rely on it as legal advice. We DO NOT have an attorney-client relationship. You should contact an attorney in your area for a one-on-one consultation before pursuing any action or making any decisions.